Fourier restriction estimates based on $L^q$-dimensions: beyond Stein--Tomas
基于 $L^q$ 维数的傅里叶限制估计:超越 Stein--Tomas
Marc Carnovale, Jonathan M. Fraser, Ana E. de Orellana
AI总结 本文提出一种新的傅里叶限制定理,用 $L^q$ 维数替代 Frostman 条件,得到连续范围的估计,在端点恢复 Stein--Tomas 结果,并部分解决 Bak 和 Seeger 的问题。
Comments 18 pages, 4 figures
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著名的 Stein--Tomas 限制定理给出了球面上曲面测度 $L^p\to L^2$ 限制估计成立的 $p$ 的精确范围。Mockenhaupt、Mitsis 以及 Bak--Seeger 将其推广到满足特定傅里叶衰减和 Frostman 条件的任意测度,最一般版本现已成为调和分析的基本结果。Frostman 条件本质上要求对小球的测度进行一致控制,它是描述测度局部波动的一系列更精细条件的端点。这种分析引出了测度的 $L^q$ 维数,它是分形几何的核心概念,也是多重分形分析和大偏差理论的关键工具。本文证明了新的傅里叶限制定理,使用 $L^q$ 维数代替 Frostman 条件,从而提供了一系列连续的估计,并在端点恢复了 Stein--Tomas 结果。我们的证明通过 Stein 的复插值给出了所有 $q\in(1,\infty]$ 的端点估计。特别地,在 $q=\infty$ 情形下,这部分解决了 Bak 和 Seeger 提出的问题。我们探讨了定理何时优于 Stein--Tomas,即范围不在 $q=\infty$ 处达到最优,并表明这相当普遍,包括某些 Mandelbrot 级联测度和具有多重分形行为的测度。在证明主要定理的过程中,我们基于某些卷积范数得到了 $L^q$ 维数的新描述,这本身可能具有独立意义。
The well-known Stein--Tomas restriction theorem gives the sharp range of $p$ for which $L^p\to L^2$ restriction estimates hold for the surface measure on the sphere. This was generalised to arbitrary measures satisfying certain Fourier decay and Frostman conditions by Mockenhaupt, Mitsis, and Bak--Seeger, with the most general version now a fundamental result in harmonic analysis. The Frostman condition essentially asks for uniform control on the measure of small balls and is the endpoint of a continuum of more nuanced conditions which describe the local fluctuations of the measure. This analysis gives rise to the $L^q$-dimensions of a measure and these are a central concept in fractal geometry and a crucial tool in multifractal analysis and the theory of large deviations. In this paper we prove a new Fourier restriction theorem which uses the $L^q$-dimensions instead of the Frostman condition, thus providing a continuum of estimates which recover Stein--Tomas at the endpoint. Our proof gives the endpoint estimate for all values of $q\in(1,\infty]$ via Stein's complex interpolation. In particular, in the case $q=\infty$ this partially resolves a question raised by Bak and Seeger. We explore when our theorem improves on Stein--Tomas, that is, when the range is not optimised at $q=\infty$, and show that this is the case quite generally, including for certain Mandelbrot cascade measures and measures with multifractal behaviour. On the way to proving our main theorem we obtain a novel description of the $L^q$-dimensions based on certain convolution norms, which may be of interest in its own right.